With increasing frequency, systemic therapies are being replaced by local ones, since in this way high levels of active substances can be attained in regions of the body which are difficult to reach, without the necessity of applying or administering high, often toxic, chemotherapeutic doses over the whole body.
DE-C-20 22 117, 25 11 122 and 27 27 535 describe bone cements based on polymethyl methacrylate (PMMA) which contain an antibiotic such as gentamycin. Additionally, other antibiotic-containing bone cements based on PMMA are known. Thus, for example, EP-A-O 301 759 describes PMMA bone cements which contain erythromycin and DE-A-35 42 972 and 35 43 164 describe PMMA bone cements which contain so-called gyrase inhibitors. It is also known to use implants based on the aforementioned PMMA bone cements in a pre-finished form for local antibiotic therapy, reference being drawn, for example, to DE-C-26 51 441 and 27 27 535.
The release of gyrase inhibitors from ceramic carrier materials such as tricalcium phosphate is also described, for example, in DE-A-35 42 972.
Furthermore, it is known to add cytostatics, for example methotrexate, to "bone cements" based on PMMA (see e.g., H. Wahlig, E. Dingeldein in "Primar- und Revisionsalloarthroplastik", published by Endo-Klinik, Hamburg, Springer-Verlag, Berlin 1987, page 357).
Since the early 70's glass ionomer cements have been used in dental medicine as filling materials and also for securing crowns and bridges (e.g., DE-A-20 61 513). In such processes the materials harden by reaction of the basic glass powder with a polymeric polyacids which is accompanied by the formation of a highly polar calcium-aluminum polysalt matrix. The use of glass ionomer cements in bone cement is likewise knowns (see e.g., L. M. Jonck, C. J. Grobbelaar, H. Strating, Clin. Mat. 4. 85 (1989)). Finally, it is known to use glass ionomer cements in a porous form as bone replacement materials (see e.g., DE-A-38 06 448).
The use of PMMA bone cements as active substance depots produces various disadvantages. For example, the materials still contain even in their hardened forms, perceptible amounts of health-damaging monomers, particularly methyl methacrylate, as well as unreacted peroxides and other radical formers. Moreover, when unhardened material is inserted for cementing purposes, a setting temperature of up to 80.degree. C. is reached during setting, which can destroy adjacent body tissue. Furthermore, the release of active substance from such bone cements (and the implants produced therefrom) is satisfactory in only a very few selected cases. For example, it is desirable to have depots provide an initially high release, followed by a subsequent continuous release of smaller doses within 2 to 30 days; in ideal situations, after this period, no more active substance should be discharged. With active substance depots based on PMMA, this could hitherto not be satisfactorily achieved.